The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the spent shale remains in place, reducing the chance of surface contamination and the requirement of disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by forming an in situ oil shale retort containing a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method of forming an in situ oil shale retort is described in U.S. Pat. No. 4,043,595, which is incorporated herein by this reference. According to U.S. Pat. No. 4,043,595, an in situ oil shale retort is formed by excavating a first portion of the formation from within the boundaries of the in situ oil shale retort being formed to form a void, where the surface of the formation defining the void provides at least one free face extending through the formation within the boundaries. A second portion of the formation is explosively expanded toward the void to form the in situ oil shale retort containing a fragmented permeable mass of formation particles. The fragmented permeable mass in the retort has a void fraction which is equal to the ratio of the volume of the void to the combined volume of the void and the space occupied by the second portion of the formation. As used herein the term "void fraction" refers to the ratio of the volume of the voids or spaces between particles in the fragmented mass to the total volume of the fragmented permeable mass of particles in an in situ oil shale retort. For example, in a fragmented mass with a void fraction of 20% , 80% of the volume is occupied by particles, and 20% is occupied by the spaces between particles.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as an oxygen-supplying gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the retort inlet mixture into the retort, the combustion zone is advanced through the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products, including gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process, is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
The residual carbonaceous material in the retorted oil shale can be used as fuel for advancing the combustion zone through the retorted oil shale. When the residual carbonaceous material is heated to its spontaneous ignition temperature, it reacts with oxygen. As the residual carbonaceous material becomes depleted in the combustion process, the oxygen penetrates farther into the oil shale retort where it combines with remaining unoxidized residual carbonaceous material, thereby causing the combustion zone to advance through the fragmented oil shale.
The rate of retorting of the oil shale to liquid and gaseous products is temperature-dependent, with relatively slow retorting occurring at 600.degree. F., and relatively rapid retorting of the kerogen in oil shale occurring at about 900.degree. F. and higher temperatures. As the retorting of a segment of the fragmented oil shale in the retorting zone progresses, and less heat is extracted from the gases passing through the segment, the combustion gas heats the oil shale farther on the advancing side of the combustion zone to retorting temperatures, thus advancing the retorting zone on the advancing side of the combustion zone.
The rate of advancement of the combustion zone through the fragmented mass depends upon the rate at which gas is introduced to the combustion zone. When gas is introduced to the combustion zone at a slow rate, the combustion zone advances through the fragmented mass slowly, and shale oil is recovered from the retort slowly. Therefore, the capital costs for preparing and operating an in situ oil shale retort are only slowly recovered.
However, if the rate of introduction of gas to the combustion zone is excessively high, a portion of the shale oil produced in the retorting zone can be consumed by reaction with oxygen passing through the combustion zone into the retorting zone. Furthermore, a high rate of introduction of gas to the combustion zone can result in a high pressure drop along the length of the fragmented mass. Therefore, blowers or compressors used for inducing gas flow through the fragmented mass will operate at relatively high pressure (for example, 5 psig), which requires appreciably more energy for driving the blowers than if the pressure drop is relatively low. The total energy requirements can be relatively high, because a long time can be required for retorting, i.e., 120 days or more. Higher pressure operation also can take a greater capital expenditure for blowers or compressors. Furthermore, some gas leakage from the retort can occur.
Also affecting pressure drop along the length of the fragmented mass is the void fraction of the fragmented mass and the average size and size distribution of particles in the fragmented mass. As the void fraction decreases or the average particle size increases, the pressure drop across the fragmented mass increases. Conversely, as the void fraction increases and the average particle size decreases, pressure drop across the fragmented mass decreases.